The present invention relates to a dual coil eddy current probe for detecting geometrical differences especially as related to threaded apertures and fasteners or studs, and a method for detecting geometric differences using a dual coil eddy current probe.
Several hundred apertures are drilled and tapped in a typical automobile engine and the various components associated with the engine. Additional apertures are required when other drive train components are included, such as suspension components, various brackets and levers, frame and sheet metal parts and the like. The process of drilling apertures and tapping or roll head formed threads in engine and drive train components does not result in perfect parts one hundred percent of the time. Quality control measures are required to test and verify the accuracy of the apertures and threads, either through a statistical sampling methodology, or through one hundred percent inspection of each drilled and tapped aperture or roll head formed thread. In many manufacturing environments, one hundred percent testing of all drilled and tapped apertures or roll-formed threads is impossible due to time constraints. In such instances, an aperture may be tapped with less than the minimum number of threads, or a damaged tap may create poorly formed threads, or worse yet, an aperture may not get tapped at all. In some instances, a tap can break off in the aperture. When such a component is transferred to the next operation or department, or worst of all, to the customer, substantial problems and costs inevitably result.
Relatively few machine builders have recognized the need for one hundred percent thread inspection following tapping operations, and most leave the inspection responsibility to the engine and drive train component manufacturers. Component manufacturers have dealt with the need to inspect threaded apertures in a variety of ways ranging from visual inspection by an operator to various forms of automated systems. Such automated systems are typically retrofitted as part of the tapping equipment, or immediately following the tapping operation as a separate step. If tapping equipment is originally equipped with thread detection, or if retrofitted, no loss in productivity occurs. If threads are detected off the production line, additional effort may be required.
With an increasingly quality driven environment at all levels of manufacturing in the world, the first efforts to inspect threaded apertures involve visual inspection by an operator of bolts or gauges actually threaded into the aperture as part of the inspection process. It quickly became evident that the human element could not be relied on or simply could not visually perform the necessary inspection due to lapse of attention, aperture depth, and so forth. Furthermore, threading a gauge into every aperture by hand was impractical at required production rates as well as relying on the human element to both read the difficult scales on such devices and to make consistent judgements as to acceptable threading torque thresholds. Enormous manpower was required to thread a bolt into every aperture by hand and this proved economically infeasible. It was recognized that a fast, consistently reliable approach was needed to inspect one hundred percent of the apertures for proper threads. Several contact and non-contact methods were developed involving quite different technologies ranging from mechanical feelers to reflective light to airflow/back pressure to electro-magnetic inspection.
Mechanical feelers are occasionally used to contact the inside edge of the aperture in order to sense movement when the feeler is moved actually within the aperture. This method determines that there are threads present on at least one side of the aperture, but does not determine much more than that. Therefore, this method has limited applications, and is not desirable for most inspection purposes.
Reflected light is another method used to detect threads. This is a non-contact, relatively quick inspection technique. It typically uses a sender/receiver probe that can be placed at an angle close to perpendicular to the face of the threads. As a light beam is directed toward the threads, the newly machined or formed surface reflects the light back to the receiver. If a sufficiently high reflectivity is present, a determination is assumed that a machined or formed surface representing threads is present in the aperture with the added assumption that the tap or roller former did the required work. With the ever present abundance of cutting fluids and coolants as well as washing fluids, both the sender and receiver units can degrade in performance. As both sending optics and receiving optics become obscured degrading optical transparency. Varying levels of reflectivity can affect detection capability when threads have fluids or oils deposited on the threads. Also, looking at only a small portion of the threaded aperture and assuming that properly formed threads are present in the rest of the aperture is not accurate in many cases, and does not form an adequate basis for accepting the aperture.
Another approach involves the use of an airflow/back pressure measurement. In this method, a probe is inserted into the aperture and injects air into the aperture. A sensor measures the back pressure of this flow as the air escapes. The turbulence created by the presence of threads causes an increase in back pressure over the back pressure present in the absence of threads, and thus the presence or absence of threads can be inferred. These systems are not adept at determining the actual number of threads to any degree of accuracy.
With the advent of smaller, better performing cameras and software, vision has been adapted for use in detecting the presence of threads in apertures. The environment of cutting fluids and oils on the cutting threads and within the aperture has a pronounced effect on the sensitive lighting requirements of the vision systems. In addition, not all threads are typically viewed with a vision system, and the assumption is made that the unseen threads are present and properly formed.
Another non-contact approach discloses the use of electromagnetic sensing using eddy currents. A probe having a single coil is excited at a specific frequency and is positioned in the aperture. The inductance of the coil plus the threaded aperture is sensed and compared to the inductance of the coil plus a known conforming threaded aperture. Since the inductance of the coil and aperture combination is affected by the combined geometry, among other things, similar geometries will exhibit similar inductance levels. By alternating current through a coil at a specific frequency and amplitude, and bringing the coil within close proximity of a part made of an electrically conductive material, the coil-part combination exhibits a combined inductance (measured in henries) as electrical currents known as eddy currents are induced in the surface of the part. The inductance of this part-coil combination is affected by primarily four characteristics of the material: microstructure/hardness; chemistry; temperature; and geometry. Since non-destructive testing using eddy currents is a comparative method, by placing the coil in the same position relative to another part, the inductance of this second combination is compared to the first one and differences can be detected. If the parts are of the same material at the same temperature, geometry differences can be very effectively detected. Thus, a tester using eddy current technology is calibrated using a known conforming part or master and then is used to detect differences in other materials relative to the geometry. Depending on the sophistication of the electronics sensing these inductance differences, more than just the presence or absence of threads can be detected. A simple, un-tuned type probe transducer circuit can, under ideal conditions, detect threaded conditions where as little as two or three threads are missing. Furthermore, the probes are unaffected by cutting fluids and coolants. The method is non-contact. The method detects 360xc2x0 of geometry and is very quick to perform. Since this method involves inspecting the aperture at a discreet position within the aperture, the probe may inspect only those threads in the immediate vicinity of the coil on the probe, and ignores the other areas of the aperture that are supposed to be properly threaded.
Another use of eddy current technology involves the use of significantly different electronics and a tuned transducer probe. This approach was developed within the last few years and is purported to have between three to ten times the sensitivity compared to the untuned version. The probes are unaffected by cutting fluids. The method is non-contact, inspects in 360xc2x0 of geometry, and is very quick to perform, approximately less than 0.1 second for the actual electronics and one to two seconds for a total cycling of the fixture actuator. The probe may not inspect threads in all parts of the aperture depending on the depth of the aperture. This method has the added advantage of being able to reliably detect one thread missing conditions in many applications due to the greater sensitivity. In most applications, the probe is advanced into the aperture while being held in a fixture with the part properly located. The fixture can use pneumatic actuators for the purpose of quickly moving the probe into and out of the aperture as controlled manually or via a programmable logic controller. The probe is positioned at the optimal depth within the aperture during each inspection in order to maximize the performance of the tuned transducer probe.
Another recent development in profiling the threaded aperture is an analog eddy current signal obtained as a function of depth into the aperture usually measured from the pierced surface. As the probe first enters the aperture, then passes through such features as counter-bores, then the threaded area itself, then into the unthreaded area of the pilot hole, the signal is continuously compared against an expected profile of a properly formed aperture. The probing coil can be concentric with the aperture centerline or it can be at 90xc2x0 to the aperture centerline. In this second situation, the process gives up the ability to detect features in a full 360xc2x0 of geometry, but can easily detect individual threads and provide the ability to count the threads. By alternating current (A.C.) coupling the probe, part to part variability challenges can be effectively negated. However, when threads need to be detected for a full 360xc2x0 geometry, another technique must be relied on.
To further improve on the single coil probe approach, the present invention includes a dual coil element probe and support electronics. This provides enhanced electromagnetic thread detection. While the above-described, tuned probe, eddy current method has excellent performance for the vast majority of applications, in a few of these applications the conforming part population exhibits a greater spread of eddy current signatures. This is due to part-to-part variations in chemistry, microstructure/hardness, and possibly temperature and/or porosity. In these situations, while using a single coil, tuned probe, it can be more challenging to separate the non-conforming parts from the conforming part population, especially if relatively minor defects are to be found. By using a tuned, dual coil probe, differences in part-to-part chemistry and microstructure/hardness are very effectively eliminated, as are temperature variations. The present invention provides a greatly improved ability to ignore conforming part variations where function is not compromised, while at the same time, retaining an extremely effective ability at detecting thread depth, thread quality, and part porosity. The dual coil probe can be positioned at a specific depth where the probe will not only sense the presence of threads, but the thread quality and the thread depth to within one thread or less, or the probe can be combined with a linear variable differential transformer (LVDT) to sense the relative position of the dual coil probe with respect to the pierce surface of the part. This configuration provides full aperture profiling capability to the user. All features, such as counter bore presence, counter bore depth, and thread quality throughout the aperture including thread depth can be detected. The present invention allows detection of these characteristics while eliminating concern about occasional variations in materials and temperatures that can sometimes limit such detection capabilities. An advantage of the dual coil probe according to the present invention is in determining the depth of threads in a part population that can vary with respect to temperature, chemistry, and microstructure/hardness, or where frequent re-calibration of the tuned, single coil probe is not likely to be performed. The dual coil probe can accomplish inspection of the part population to within less than one thread, consistently and without frequent re-calibration.
Eddy current thread detection systems use both the phase shift and amplitude change of the electromagnetic signature that is induced in a sample by an inductor (transducer) to create a xe2x80x9ctransducer voltage.xe2x80x9d This transducer voltage relates basically to the electrical conductivity of a material and represents four material characteristics including: geometry; chemistry; case depth hardness/microstructure; and temperature. When the transducer is placed in an aperture which conforms to specifications, the transducer voltage is xe2x80x9clearnedxe2x80x9d by the thread detection system electronics so that the xe2x80x9clearnedxe2x80x9d transducer voltage can be used to compare it with other transducer voltages associated with apertures to be tested. While the transducer is properly positioned in an aperture, the detection system electronics creates an internal compensating voltage causing the transducer voltage to approach zero volts. The magnitude of the compensating voltage is stored in digital memory. As the transducer is placed in subsequent apertures, the compensating voltage is recalled and combined with the new transducer voltage. Each new sample voltage is displayed on the channel volt meter and compared to a threshold around the zero volt reference created during the learning process. The xe2x80x9cwindowxe2x80x9d adjustment allows the user to establish two thresholds around the xe2x80x9czero voltsxe2x80x9d reading with one threshold being positive in voltage and the second equally negative in voltage. If the transducer voltage of each new aperture falls between these two threshold values, the channel indicates conformance with the learned sample. If the transducer voltage is outside the two thresholds, the channel indicates non-conformance with the learned sample.
The present invention relates to the inspection of threaded apertures and threaded studs and is primarily used for quality control to evaluate the integrity of threads in or on electrically conductive parts. According to the present invention, parts are located and positioned relative to a probe. The probe includes two or more separate coils wound around an axis of the probe tip. The coils preferably are separated by a distance less than the depth or length of a threaded region to be inspected. The probe is moved into the aperture or onto the stud approximately along a centerline of the stud, so that the probe passes into the threaded region. The coils have a known electrical inductance and are electrically excited at a known frequency and amplitude. As the probe moves into and through the aperture or onto the stud, the amplitude and phase of the excitation voltage is extracted and measured for each coil as eddy currents are induced in the surrounding, electrically conductive material. These voltage amplitude and phase measurements can then be collectively or individually compared to analogous measurements taken while inspecting another part. The voltage amplitude and phase measurements can be subtracted and analyzed. The individual, collective, and /or subtracted measurements can be analyzed at a discreet point along the path of the probe during the inspection process, or the measurements can be analyzed on a continuous and/or segmented basis. The inspection process, according to the present invention, can include a linear variable differential transformer (LVDT) or linear potentiometer to measure the position of the probe along the axis, depth, or length of the aperture or stud. The apertures and studs can include tapped or machined threads, roll-formed threads, or blind apertures, through apertures, counter bores, and any combinations thereof. The present invention includes a probe with two or more separate coils and electronics sufficient to analyze the various amplitude and phase measurements. The present invention can include a linear variable differential transformer or linear potentiometer to measure depth or distance of probe travel and can also include a device to analyze the various amplitude and phase measurements on a continuous and/or segmented basis.
It is desirable in the present invention to provide a substantial improvement in the ability to distinguish between parts that are threaded to the fully desired depth or length and those that are threaded to a shorter or longer depth or length then possible using a probe with a single coil as previously known. It is desirable in the present invention to compare the various amplitude and phase measurements of one coil to the same measurements from one or more other coils in or on the same part. It is desirable in the present invention to provide more than one coil in the part region being inspected simultaneously in order to make a comparison between the coils. It is desirable in the present invention to be able to distinguish between conforming and non-conforming parts within a population of parts that are threaded to the fully desired depth or length while exhibiting variations in temperature, chemistry, and/or microstructure or hardness. The ability of a single coil probe and support electronics to compare a population of parts to a master part on the basis of those parts which are similarly threaded and those with less than the desired thread or depth of threads is limited because the various amplitude and phase measurements of the coil can be affected by the previously mentioned non-geometrical variations, such as temperature, chemistry, microstructure, and/or hardness. The dual coil probe, according to the present invention, effectively reduces or eliminates non-geometrical part-to-part variations and the comparison of measurements taken from one coil to those of the other coil or coils when both coils are used to inspect the same region of the same part.
Other objects, advantages and applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.